Power steering apparatuses are widely used as an apparatus for reducing the force required for operating a steering wheel when applying a steering angle to the steered wheels of a vehicle. Electric-powered power steering apparatuses that use an electric motor as an auxiliary power source are already widespread. Reduction gears are assembled in such electric-powered power-steering apparatuses, and worm reduction gears having a large lead angle and having reversibility in the power transmission direction are typically used for this kind of reduction gear.
FIG. 4 and FIG. 5 illustrate an example of a conventional worm reduction gear as disclosed in JP 2009-072841 (A). This worm reduction gear has: a worm 4 that is fastened to an electric motor 1, is located on the inside of a housing 2 for the worm reduction gear, and has a worm tooth 3 formed around the middle section in the axial direction of the worm 4; and a worm wheel 5 that engages with the worm tooth 3. The worm 4 is supported by a pair of ball bearings that fit around both end sections in the axial direction so as to be able to freely rotate inside the housing 2, one end of the worm 4 (left end in FIG. 4) is connected to the output shaft 7 of the electric motor 1, and the worm 4 is rotated and driven by the electric motor 1.
The worm wheel 5 is located inside the housing 2 so as to rotate freely and so that the center axis of rotation of the worm wheel 5 is in a skewed position with respect to the worm 4. The tooth section 8 that is formed around an outer circumferential edge portion of the worm wheel 5 and the worm tooth 3 engage and make it possible to transmit torque between the worm 4 and worm wheel 5. The worm wheel 5 is fastened around the outside of the middle section of the steering shaft 9. With this kind of construction, the rotational driving force that is generated by the electric motor 1 can be transmitted to the steering shaft 9 by way of the worm 4 and worm wheel 5.
The threaded section (helical structure) of the worm tooth of this kind of worm is formed by performing a cutting process or grinding process on the outer circumferential surface or inner circumferential surface of a rod-shaped object or tubular-shaped object. As this kind of threaded section, instead of the worm tooth of a worm, there is also the ball-screw groove of the ball-screw shaft of a ball-screw device, the female screw section of a nut and the like.
FIG. 6 illustrates an example of a conventional threaded section grinding device for forming a screw groove on the outer-circumferential surface of a rod-shaped member as disclosed in JP H09-323218 (A). This grinding device has a first drive device 10, a rotary drive shaft 11, a grip section 12, measurement means 13, a machine tool 14, a second drive device 15, an axial feeding device 16 and a controller 17.
The first drive device 10 has a servo motor, and is mounted on one side of the top surface of a feeding table 18. The feeding table 18 is provided so as to be able to be displaced in the axial direction of a machined article 19 by the axial feeding device 16. Unless stated otherwise, the axial direction is the axial direction of the machined article. The rotary drive shaft 11 is rotated and driven by the first drive device 10. The grip section 12 is provided on the tip end of the rotary drive shaft 11, and one end of the machined article 19 can be concentrically fastened to the rotary drive shaft 11 so as to be able to integrally rotate with the rotary drive shaft 11.
The measurement means 13 is located on the other side of the top surface of the feeding table 18. The measurement means 13 has a tailstock 20, and is constructed so as to measure the amount of thermal expansion in the axial direction of the machined article 19 that is being machined in a state that the tip end of the tailstock 20 is in contact with the other end of the machined article 19. The detailed construction of the measurement means 13 is known, such as disclosed in JP H9-323218 (A), so an explanation thereof will be omitted.
The machine tool 14 has a grindstone and is used for machining the outer-circumferential surface of the machined article 19. The second drive device 15 has a servo motor, and drives the axial feeding device 16. The axial feeding device 16 has a feed-screw mechanism, and by being driven by the second drive device 15, causes the feeding table 18 to move in the axial direction. By the feeding table 18 being displaced in the axial direction in this way, it is possible for the machine tool 14 to move relative to the machined article 19 in the axial direction. Furthermore, the controller 17 controls the rotational speed of the rotary drive shaft 11 and the feeding speed of the axial feeding device 16, or in other words controls the speed of relative displacement in the axial direction between the machine tool 14 and the machined article 19 by controlling the first drive device 10 and the second drive device 15.
This kind of grinding device is able to form a screw groove on the outer-circumferential surface of a machined article 19 by causing the first drive device 10 to rotate the rotary drive shaft 11 and the machined article 19 and causing the second drive device 15 to cause relative displacement between the machine tool 14 and machined article 19.
When machining a threaded section using this kind of grinding device, the machined article 19 undergoes thermal expansion as the temperature of the article 19 being machined increases. This kind of thermal expansion causes variations to occur in the lead of the threaded section that is formed on the machined article 19. More specifically, when machining is performed with a constant rotational speed of the machined article 19 with respect to the machine tool 14 and constant feeding speed in the axial direction of the machined article 19 and the lead of the threaded section that is formed during the first half of machining is compared with the lead of the threaded section that is formed during the latter half of machining, the lead of the threaded section that is formed during the latter half when the amount of thermal expansion is large becomes small. In other words, a threaded section that is formed on a fixed lead regardless of whether the amount of thermal expansion is large or small is such that the thermal expansion is constricted and decreases the more the lead section of the threaded section is formed on a portion where the amount of thermal expansion is large.
On the other hand, this grinding device is constructed so as to send a signal indicating the amount of thermal expansion of the machined article 19 that was measured by the measurement means 13 during machining to the controller 17 by way of an amp 21, and so that the controller 17 calculates a correction value for the drive force of the first drive device 10 (rotational speed of the rotary drive shaft 11) and the drive force of the second drive device 15 (feeding speed of the axial feeding device 16) based on information that was sent from the measurement means 13, and sends a new instruction (amount of drive) that was calculated based on the correction values to the first drive device 10 and second drive device 15. With this kind of construction, it is possible to correct the machining conditions for machining the machined article 19 in real-time based on the amount of thermal expansion of the machined article 19 that was measured during machining, so variation of the lead of the threaded section that is formed on the machined articles 19 is prevented.
This grinding device is effective in keeping the lead of the threaded section that is formed around the outer circumferential surface of the machined article having a long overall length and long machining time as in the case of a ball-screw rod from becoming different during the first half of machining and the latter half of machining, however, when machining a machined article having a short overall length and short machining time, there is a possibility that feedback control will not always be performed effectively and stably. For example, in the case of a machined article having a relatively short dimension in the axial direction, and short machining time, such as in the case of a worm, it is necessary to perform highly precise feedback control at high speed in order to keep the amount of difference in the lead of the threaded section that is formed around the outer-circumferential surface of the machined article during the first half and the latter half of machining of this machined article small.
However, in the case of feedback control in real-time of a machined article having a short machining time such as this, error in the measurement value for feedback control, or noise that is mixed in with that measurement value has a large effect on the machining conditions, and it may become difficult to perform effective feedback control. Therefore, in order to obtain a suitable effect from this kind of feedback control, expensive equipment (measurement device, CPU of a controller, drive devices and the like) is necessary. Moreover, the machining device illustrated in FIG. 6 is a grinding device, however, in the case of machining device for performing cutting device, the machining speed is faster than in grinding device, so the machining time for each machined article becomes short, and there is a possibility that the same problem will occur. Therefore, in the case of a machining device for performing cutting as well, expensive equipment is necessary in order to perform feedback control effectively and stably.